The present invention relates to a method for monitoring the position and orientation of a moving object, of the type in which the moving object transmits electromagnetic signals, representative of the position and orientation thereof, to a fixed receiver. More particularly, the present invention relates to an open loop method in which either the transmitter or the receiver may be spatially extended and in which both the position and the orientation of the moving object are computed noniteratively.
It is known to track the position and orientation of a moving object with respect to a fixed frame of reference, by equipping the moving object with a transmitting apparatus that transmits electromagnetic radiation, placing a receiving apparatus in a known and fixed position in the fixed frame of reference, and inferring the continuously changing position and orientation of the object from signals transmitted by the transmitting apparatus and received by the receiving apparatus. Typically, the transmitting apparatus includes three orthogonal magnetic dipole transmitters; the receiving apparatus includes three orthogonal magnetic dipole receivers; and the object is close enough to the receiving apparatus, and the frequencies of the signals are sufficiently low, that the signals are near field signals. Also typically, the system used is a closed loop system: the receiving apparatus is hardwired to, and explicitly synchronized with, the transmitting apparatus. Representative prior art patents in this field include U.S. Pat. No. 4,287,809 and U.S. Pat. No. 4,394,831, to Egli et al.; U.S. Pat. No. 4,737,794, to Jones; U.S. Pat. No. 4,742,356, to Kuipers; U.S. Pat. No. 4,849,692, to Blood; and U.S. Pat. No. 5,347,289, to Elhardt. Several of the prior art patents, notably Jones, present non-iterative algorithms for computing the position and orientation of magnetic dipole transmitters with respect to magnetic dipole receivers.
Of particular note are U.S. Pat. No. 4,054,881, to Raab, and U.S. Pat. No. 5,600,330, to Blood. Raab purports to teach an open loop system. Raab's system is "open loop" only in the sense that there is no communication from the receiving apparatus to the transmitting apparatus; but it still is necessary to synchronize the transmitting apparatus and the receiving apparatus explicitly. Raab provides several methods for synchronizing the receiving apparatus with the transmitting apparatus, for example a phase locked loop in the case of frequency domain multiplexing, and code timing signals, in the case of spread spectrum multiplexing. In all cases, however, Raab's system requires that the receiver generate a reference signal that is mixed with the received signal, both for the purpose of synchronization and for the purpose of resolving sign ambiguities in all three independent coordinates of the space in which the object moves. In Blood's system, the transmitters are fixed in the fixed reference frame, and the receivers are attached to the moving object; but by reciprocity, this is equivalent to the situation in which the receivers are fixed and the transmitters move. Blood's transmitters are spatially extended, and so cannot be treated as point sources. Blood also presents an algorithm which allows the orientation, but not the position, of the receivers relative to the transmitters to be calculated non-iteratively.
It thus is apparent that there is further room for simplification of the art of tracking a moving object using near field electromagnetic signals. The explicit synchronization required by Raab demands additional hardware and/or signal processing that would not be necessary if explicit synchronization were not required. Blood's iterative calculation of position adds complexity and processing time, to systems with spatially extended transmitters or receivers, that are absent from systems with point sources and point receivers. It would be highly advantageous to have a noniterative method of inferring both the position and the orientation of a transmitting apparatus relative to a spatially extended receiving antenna without explicit synchronization of the transmitters and the receivers.